Definitions of mouthpiece:

• noun:   the aperture of a wind instrument into which the player blows directly
• noun:   the tube of a pipe or cigarette holder that a smoker holds in the mouth
• noun:   an acoustic device; the part of a telephone into which a person speaks
• noun:   (especially boxing) equipment that protects an athlete's mouth
• noun:   a part that goes over or into the mouth of a person

  Example: "The mouthpiece of a respirator"

• noun:   a spokesperson (as a lawyer)